Low temperature formation of highly resistive ZnO films using nonequilibrium N2/O2 plasma generated near atmospheric pressure

Abstract

Low temperature chemical vapor deposition of ZnO thin films was demonstrated using β-diketonate complexes [Bis-2,4octanedionato zinc: Zn(OD)2] and nonequilibrium N2/O2 plasma generated near atmospheric pressure. ZnO films grown at the substrate temperature of 200°C and at the O2 concentration in the N2/O2 mixture gas (O2%) of 0.2, 3.8 and 20% exhibited excellent (0001) preferred orientation on glass substrates, high transmittance of above 85% in the visible region and smooth surface structure with the continuous columnar grains. Electrical measurement revealed that our ZnO films were highly resistive with the specific resistivity exceeding 106Ωcm at room temperature, which is not easily obtained by the conventional high-vacuum processes without intentional acceptor doping. Thermally stimulated current measurement detected two peaks from highly resistive ZnO film grown at the O2% of 0.2%, which correspond to the carrier emission from deep trap levels with the activation energies of 170 and 610meV. The former was assigned as zinc vacancy (VZn) or nitrogen substituting oxygen site (NO), and the latter was assigned as VZn or oxygen interstitial (Oi). The carrier compensation by these deep acceptor levels seems to be the origin of high resistivity.